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Capturing Durability of High Recycled Binder Ratio (RBR) Mixtures

Description:

Environmental sustainability and economic benefits motivate maximum use of recycled asphalt materials, including reclaimed asphalt pavement (RAP) and recycled asphalt shingles (RAS), in asphalt mixtures. However, adequate cracking performance initially and with time when these stiff, brittle, aged materials are added to virgin asphalt-aggregate mixtures must be maintained by engineering each unique materials combination of virgin and recycled materials. To preclude cracking, state departments of transportation (DOTs) currently limit the RAP and RAS contents and overall recycled binder ratio (RBR) from these materials. Strategies for partially restoring rheology and improving cracking performance include utilizing a softer base binder or adding a recycling agent (rejuvenator), but the durability of these strategies with aging and exposure to moisture is unknown. In addition, these types of high RBR asphalt mixtures must be included in balanced mix design systems currently being implemented. NCHRP 09-58 The Effects of Recycling Agents on Asphalt Mixtures with High RAS and RAP Binder Ratios produced a draft AASHTO Standard Practice for high, 0.3-0.5, RBR asphalt mixtures that includes:

  1. component materials selection guidelines for virgin binders, recycling agents, and recycled materials and their proportions;2. a recycling agent dose selection method;3. a recycled binder availability factor for mix design;4. binder blend rheological evaluation tools; and5. mixture performance evaluation tools.

The evaluation tools include aging protocols, recycling agent blending methods, and laboratory tests and corresponding thresholds for adequate rutting and cracking performance.

The draft AASHTO Standard Practice was developed based on five field projects in TX, NV, IN, WI, and DE; each with multiple test sections but none with recycling agents at higher doses selected by the method developed to maintain rutting resistance while maximizing cracking resistance with aging based on laboratory tests. The guidelines and tools in the draft AASHTO Standard Practice allow for evaluation of rutting and cracking resistance for a specific combination of materials, but moisture susceptibility was not addressed in detail but is of critical concern with rejuvenation by recycling agents. Aging protocols associated with the evaluation tools also likely need to be updated based on the results of NCHRP 09-54 Long-Term Aging of Asphalt Mixtures for Performance Testing and Prediction and NCHRP 09-61 _Short- and Long-Term Binder Aging Methods to Accurately Reflect Aging in Asphalt Mixtures _and the continued challenge of practically capturing field aging in different climates in the laboratory, especially for modified binder blends and mixtures. In addition, further verification of the cracking performance thresholds in the draft AASHTO Standard Practice can be realized by continued monitoring of the five field projects from NCHRP 09-58.

Objective:

The objective of this research is to expand and update the draft AASHTO Standard Practice produced in NCHRP 09-58 for high RBR asphalt mixtures to capture durability with:

  • tools to evaluate moisture susceptibility* updated aging protocols* verified cracking performance thresholds
Benefits:

A comprehensive standard practice and/or system is needed to specifically address the complexity of engineering durable high RBR asphalt mixtures with balanced performance to promote environmental sustainability of the most recycled product in the U.S. and realize significant economic benefits. An economic analysis conducted as part of NCHRP 09-58 indicates that doubling RAP contents from 20 to 40% with the use of recycling agents at doses up to 8-10% can result in savings from $6 to $8 per ton HMA, or from $0.30 to $0.40 per one percent RAP, or 7-10% of in-place HMA price when virgin materials costs are relatively high.

In addition to economic benefits, NAPA estimates that almost 50 million cubic yards of landfill space was saved by recycling RAP and more than $2.2 billion was saved in 2017 by using RAP and RAS instead of virgin materials. These substantial environmental sustainability and economic benefits can be realized with a comprehensive draft AASHTO Standard Practice for high RBR asphalt mixtures that includes durability guidelines and evaluation tools that consider moisture susceptibility and aging with rutting and cracking performance that can be utilized in a balanced mix design system.

Related Research:

This research complements that done in NCHRP 09-58 but expands the resulting draft AASHTO Standard Practice for high RBR asphalt mixtures to include durability guidelines and evaluation tools that align with aging protocols developed in NCHRP 09-54, NCHRP 09-52 Short-Term Laboratory Conditioning of Asphalt Mixtures and the associated NCHRP 09-52A, and ongoing NCHRP 09-61. Consideration should also be given to: (1) the moisture susceptibility evaluation guidelines developed for warm mix asphalt (WMA) mixtures in NCHRP 09-49 Performance of WMA Technologies: Stage I – Moisture Susceptibility and validated in the associated NCHRP 09-49B and (2) the recommendations for cracking tests and thresholds from NCHRP 09-57 _Experimental Design for Field Validation of Laboratory Tests to Assess Cracking Resistance of Asphalt Mixtures _and the ongoing follow-on NCHRP 09-57A. Without the specific inclusion of the durability issues of moisture and oxidative aging, only the environmental sustainability benefits are attained through the increased use of recycled materials and the engineering performance and therefore economic benefits that range from state to state to a maximum of approximately $159 million annually would not be realized and public perception of environmental sustainability efforts could be negatively impacted if the performance is inadequate.

Tasks:
  1. Evaluate current and relevant tests and methods for characterizing moisture susceptibility of asphalt mixtures including the guidelines developed for WMA in NCHRP 09-49 and validated in the follow-on NCHRP 09-49B.
  2. Review aging protocols developed in NCHRP 09-54, NCHRP 09-52/09-52A, and NCHRP 09-61 and binder aging modeling results from NCHRP 09-58.
  3. Define practical parameters to capture field aging of high RBR asphalt mixtures in the laboratory that include initial quality and rate of aging.
  4. Further verify cracking performance thresholds developed in NCHRP 09-58 by monitoring established field projects and others associated with implementation efforts in Texas, Illinois (and possibly other states including Minnesota and California) and evaluate alternate cracking tests and thresholds recommended in NCHRP 09-57/09-57-A.
  5. Expand and update the draft AASHTO Standard Practice for high RBR asphalt mixtures produced in NCHRP 09-58 to include durability in terms of moisture susceptibility and aging.
Implementation:

The results of this research will be used by highway agencies, asphalt mixture designers, and asphalt paving contractors to produce durable asphalt mixtures for pavements with high RBR that are environmentally sustainable and economically viable. Implementation activities may include:

  • review of the expanded and updated draft AASHTO Standard Practice for high RBR asphalt mixtures by national task groups for comment and identification of champions for national implementation and
  • a national workshop with invited participants for briefing on the expanded and updated draft AASHTO Standard Practice for high RBR asphalt mixtures
Relevance:

This research will result in a comprehensive draft AASHTO Standard Practice for high RBR asphalt mixtures that includes durability guidelines and evaluation tools for moisture susceptibility and aging that can be used by highway agencies, asphalt mixture designers, and asphalt paving contractors to identify component materials combinations (virgin binder, recycled materials including RAP and RAS, recycling agent, and virgin aggregate) and their proportions where significant environmental sustainability and economic benefits can be realized without sacrificing balanced engineering performance.

Sponsoring Committee:AKM30, Asphalt Materials Selection, and Mix Design
Research Period:Longer than 36 months
Research Priority:High
RNS Developer:Amy Epps Martin, Matt Corrigan, Shane Underwood
Source Info:Reports and published papers and presentations from NCHRP 9-58, NCHRP 9-54, NCHRP 9-52, NCHRP 9-52A, NCHRP 9-61, NCHRP 9-57, NCHRP 9-57A, NCHRP 9-49, and NCHRP 9-49B
Asphalt Pavement Industry Survey on Recycled Materials and Warm-Mix Asphalt Usage 2017, IS 138, National Asphalt Pavement Association
Date Posted:02/19/2019
Date Modified:09/10/2019
Index Terms:Durability, Recycled materials, Bituminous binders, Asphalt mixtures, Asphalt pavements,
Cosponsoring Committees: 
Subjects    
Highways
Materials
Pavements

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